The present invention is directed to the improved quality, in uniformity of strength, softness, drapability, and textile-like feel of non-woven webs produced from continuously drawn filaments of spinnable polymeric thermoplastics. The invention relates to the controlled orientation of filaments as laid on a collector in the for of a non-woven web of a coherent structure, and to the controlled molecular orientation of the filaments themselves to provide a fabric-like material of autogenously or self-bonded filaments and fibers. This invention is especially concerned with the stabilization and control of the physical deposition of polymeric filaments on a traveling collector and with increasing the density or quantity of filament intersection points for increased filament bonding without producing any adverse effects on drapability or the soft textile-like hand of the non-woven web.
Non-woven webs comprising a plurality of substantially continuous and randomly deposited, molecularly oriented filaments of thermoplastic polymers are widely known in the art and are finding widespread commercial use. However, there is a great need for non-woven webs having a higher uniformity, better hand, greater strength add a better control of the uniformity of the molecular orientation of the individual filament than are presently available.
Until the instant invention, non-woven webs have been prepared by simultaneously spinning a multiple number of continuous filaments of a synthetic polymer such as polypropylene through a multiple number of spinning nozzles or spinnerets, preferably extending in one or more rows. The filaments are simultaneously drawn through air guns, eductors, or air jet drafters (air suckers) at high velocities in individually surrounding gas columns directed by exit nozzles to impinge on a moving collector, in loop like, overlapping arrangements, where they form a continuous non-woven random laid web which may be consolidated, compacted and stabilized by various bonding techniques such as hot calendering, autogenous spot bonding by passing the web between heated patterned embossing rolls, needle punching, or treating with suitable binders.
The filaments are drawn downwardly at velocities of approximately 600 to 8000 meters per minute in surrounding gas columns flowing at supersonic velocities and impinging on a horizontal carrier which is moving at speeds generally in the range of 150 to 300 yards per minute. This low ratio of web production capability to the filament output results in a relatively uncontrollable random laydown of the filaments with an accompanying adverse effect on the uniformity of strength, opacity, drapability, and soft fabric-like hand. After formation on the carrier, the web is passed between two rollers and lightly compacted prior to passing through the pressure nip of two heated rolls, one of which contains a plurality of raised points on its surface. The amount of prewrap and the roll temperature is critical in that too high a web temperature results in high web shrinkage, film forming effects and over-bonding with its adverse effect on drapability, and can also result in filament degradation with an accompanying reduction in filament tenacity. If the web temperature is to low, the filaments release from their bond points before any substantial strain is applied to the filaments allowing the web to slither apart.
It can be seen that the prior non-woven fabrics are produced by clumsy and quite uncontrollable processes, which also have very low ratios of filament output to web production capability, thereby increasing production costs and capital equipment dollar outlays. In addition, the prior web structures have relatively few filament intersection points, which puts limitations on the mechanical properties in that it is difficult to achieve appropriate bonding without an accompanying adverse film forming effect of the web surface and a deleterious effect on the fabric drapability and hand.
The prior webs all have one thing in common and that is that the filaments are all laid in a looplike random arrangement onto a carrier belt or the like with high velocity are to form a web. Accordingly, they are all subject to the problems associated with air formed webs, such as turbulent air flow with resultant filament intertwining, and plugging of eductors by broken filaments or molten polymer, all of which impart an undesirable non-uniformity in appearance, drapability, tensile strength, opacity, basis weight, and variations in degree of filament entanglement. Variations in the gap space of air jet slits result in non-uniform flowing action of air jets on filaments, resulting in non-uniform webs. Slight variations of the conditions for cooling destroys the uniformity of distribution of the filaments, and difficulties of getting all eductors or air channels to produce filaments having the same characteristics are manifold. The drapability is poor due to the high numbers of autogenous spot bonds required to form a coherent structure and web of commercial integrity. Also, the installation costs and maintenance expenses, and the required capital investment in air handling equipment and ducting for the high volumes of high pressure heated air required for the blasting action of the air jets on the filaments to draw and deposit them on the collecting device are immense. The above described methods require high air consumption of heated air, which in turn consumes huge amounts of power.
Illustrative prior art techniques of the production of substantially continuous filaments are described in the following U.S. Pat. Nos.: Kinney 3,338,992 and 3,341,394; Hartmann 3,502,763, 3509,009, and 3,528,129; Peterson 3,502,538; Dobo et al 3,542,615; Levy 3,276,944; and Talbert 3,506,744, as well as the illustrative techniques described in the U.S. Pat. No. 3,565,729 to Hartmann in which it is disclosed that molten polymer be subject to fusion spinning and drawing by means of directed gas currents, which seize the molten filaments from at least two sides, will produce fibers of high molecular orientation. The gas velocity is adjusted so that the filaments are carried away from the spinneret without breaking off. However, variations in polymer melt flow, melt temperature variations, gas temperature, and gas velocity have an influence on frequency of filament breakage and non-uniformity in appearance, basis weight, and degree of filament entanglement.
U.S. Pat. No. 3,692,618 to Dorschner et al teaches eductive drawing wherein discrete jets are formed which entrai a surrounding fluid in turbulent flow. The polymer melt is extruded through a multiple number of spinnerets extending in a row and are gathered into a straight row of side-by-side evenly spaced apart untwisted bundles. These filament bundles are passed through air guns and deposited on a carrier in a loop-like random arrangement.
U.S. Pat. No. 3,802,817 discloses a large number of monofilaments that are melt spun from a number of orifices and then introduced into a single-nozzle stage sucker having a narrow slit-like passage opening formed vertically through the sucker and located far enough below the orifices to coagulate at least the surfaces of the filaments. The filaments are impinged on both sides by a pair of jet air streams thereby subjecting the curtain-like arranged filaments to cold stretching and deposition on a traveling foraminous belt.
Another prior art spinning process suggests injecting high temperature and high pressure steam at a close proximity to the extrusion spinneret and upon the filaments as extruded in order to increase the filament velocity to draw them and orient the polymer molecules in the direction of the filament axis. However, injecting high temperature, high pressure, and high velocity steam on filaments as extruded leads to frequent filament breakage. This consumption of large quantities of high pressure, high temperature steam increases capital equipment costs, a well as operating costs.
A further improvement proposal suggests providing several stages of nozzles in the sucker to maintain a nearly laminar flow of the sucking and injecting gaseous medium flowing through the sucker. However, the filaments moving through the sucker become entangled, thereby affecting the web uniformity across wide webs.
Eductive type devices, whether they ar air jet drafters ejectors, air suckers, or aspirator jets, require two sources of air supply and compressing equipment, one being a low pressure, cooled air source for quenching the solidifying filament at least to the non-tacky state and the other a high pressure air source to produce high velocity air for drawing the filaments. The requirement of two sets of air compressing equipment, coupled with high precision costly machine work with its associated high installation costs, and high maintenance expenses in turn, results in high production costs of the webs.
The use of high pressure air for educting the low pressure air causes a highly turbulent flow which in turn causes filament intertwining and breakage. Associated with turbulent flow are the difficulties of getting all the air channels and eductors to produce filaments having the same characteristics, which in turn have a deleterious effect on the basis weight profiles due to poor bundle spreading and variations in filament entanglement.
The air supplied to the quench chamber must be free of secondary circulations, low in turbulence, uniform in distribution and cooler than the filaments being extruded. This approach flow must be essentially free of any large scale eddies or vortices. The non-uniformity of the filaments stream and the entanglement of the filaments is an inherent problem with prior methods of producing continuous filament random laid webs. The nozzle openings and the collection distance affect the uniformity of the final web to a high degree in the forming of the loops and migrations of the filaments. Prior equipment has difficulty getting all air channels or eductors to produce filaments having the same characteristics. Problems caused by broken filaments include the plugging of air channels or eductors.
Some of the prior apparatus and methods employ the repulsive forces developed by the application of static high voltage to filament groups, as described in U.S. Pat. No. 3,506,744, to separate large numbers of monofilaments to improve the uniformity of a blasted laydown on a foraminous belt with the use of high velocity air. This method with its associated costly and critical equipment further complicates the process.
The methods of preparing the continuous filament webs described above have at least three common features.
1. Continuously extruding a thermoplastic polymer, either from a melt or a solution, through a spinneret in order to form discrete filaments.
2. The filaments are drawn or drafted by high velocity air in order to molecularly orient the polymeric filaments and achieve tenacity.
3. The filaments are deposited in a blast of high velocity air or gas in a substantially random manner onto a carrier belt or the like to form a web with substantially isotropic physical characteristics.
It can readily be seen that the prior art has been directed towards methods and devices for eliminating processing problems and the non-uniform properties of melt spun and air grafted filaments, which after drafting or having been air drawn are blasted against a foraminous moving collector at a speed of about two to three times the spinning velocity, which in some cases would reach 6000 to 18,000 meters per minute. However, the methods and equipment used leave much to be desired with respect to heat setting of filaments under relaxed or tensile conditions, differential drawing, crimped or incremental drawing, hot or cold drawing under controlled temperature, and uniform drawing conditions while in either a crystalline or amorphous state or in both states.
Filaments that are drawn pneumatically enter a quench chamber, upon exiting from the spinneret, and are immediately drawn and solidified. The filaments are molecularly oriented but not to the extent of filaments subjected to mechanical draw down under numerous, controlled, interrelated processing variables.
Filaments that are drawn mechanically enter a heated or controlled temperature chamber upon exiting the spinneret and are drawn away from the orifice at a greater rate than the rate of extrusion to effect a substantial draw down of the filaments in the molten state prior to solidification thereof. The solidified filaments having a low degree of molecular orientation are then subjected to a mechanical draw down with draw rolls under closely controlled temperature and velocity conditions, thereby imparting a much higher degree of molecular orientation to the continuous filament than that obtained by pneumatic drawing methods.
It is well known in the art that mechanical drawing of freshly-spun synthetic filaments with draw rolls produce more uniform tensile properties from spinneret to spinneret. Until the instant invention, the molecular orientation of filaments with the use of draw rolls has not been coupled with the spinning operation in such a manner that would permit a substantially parallel laydown of filaments; that is, a web having the appearance of woven cloth on a collector in a controlled manner in a single rapid and continuous operation. The biggest obstacle to this process is that mechanical drawing of filaments with draw rolls necessitates tension on the filaments leaving the last draw roll to strip the filaments from the roll and to prevent slippage of the filaments on the draw roll. Until the instant invention, the tension was provided by various types of jet devices, which are subject to frequent and costly plug-ups.